CO2 recovery apparatus and CO2 recovery method

ABSTRACT

A CO 2  recovery apparatus according to the present invention includes: an absorber ( 1003 ) that brings CO 2 -containing flue gas ( 1001 A) into counter-current contact with CO 2  absorbent ( 1002 ) to reduce CO 2 , and a regenerator ( 1005 ) that regenerates rich solution ( 1004 ) that has absorbed CO 2 , in which lean solution ( 1006 ) having CO 2  reduced in the regenerator ( 1005 ) is reused in the absorber ( 1003 ). The absorber ( 1003 ) further includes a CO 2  absorbing unit ( 1010 ) that recovers CO 2  contained in the flue gas ( 1001 A), and the CO 2  absorbent ( 1002 ) that has absorbed CO 2  is extracted from a rich side of the CO 2  absorbing unit ( 1010 ) to exterior, cooled, and then supplied to a position nearer to a lean side of the absorber ( 1003 ) with respect to the position at which the CO 2  absorbent ( 1002 ) is extracted.

TECHNICAL FIELD

The present invention relates to a CO₂ recovery apparatus and a CO₂ recovery method for circulating CO₂ absorbent that has absorbed CO₂ contained in flue gas in an absorber to allow the CO₂ absorbent to absorb a greater amount of CO₂, thus increasing the CO₂ concentration in the CO₂ absorbent and reducing the circulation mass flow of the CO₂ absorbent, so that the amount of steam required in regenerating the CO₂ absorbent in a regenerator is reduced, resulting in saved energy.

BACKGROUND ART

The greenhouse effect of CO₂ has been pointed out as one of the causes of the global warming, and it has become an international urgent task to provide countermeasures to CO₂ to protect the global environment against the warming. CO₂ is generated by any human activities involving the combustion of fossil fuels, and there are increasing demands for suppressing emissions created thereby. Along with such an increasing demand, as countermeasures are being sought for increased productions of raw materials such as urea (a chemical application) and crude oil, and for global warming, resulting in studies energetically being made on a method for reducing and recovering CO₂ contained in flue gas by bringing the flue gas emitted from a boiler in contact with an amine-based CO₂ absorbent, and storing recovered CO₂ without releasing it into the air. Applications include power plants that consume a large amount of fossil fuels, such as a thermal plant.

As a practical method for recovering and storing a large amount of CO₂ contained in flue gas, a chemical absorption technique in which CO₂ is brought into contact with a CO₂ absorbent such as aqueous amine solution is a known method. In a method using the CO₂ absorbent, flue gas is brought into contact with the CO₂ absorbent in an absorber to reduce and to recover CO₂ contained in the flue gas, the absorbent that has absorbed CO₂ is heated in a regenerator so as to isolate CO₂ and to regenerate the absorbent, and the absorbent is circulated back to the absorber for reuse (Patent Document 1).

FIG. 10 is a schematic of an example of a structure of a conventional CO₂ recovery apparatus 1000A. As illustrated in FIG. 10, the conventional CO₂ recovery apparatus 1000A includes: an absorber 1003 in which CO₂-containing flue gas 1001A emitted from an industrial facility is brought into contact with CO₂ absorbent 1002 to reduce CO₂ contained in the CO₂-containing flue gas 1001A, and a regenerator 1005 that causes the CO₂ absorbent that has absorbed CO₂ (hereinafter, the CO₂ absorbent that has absorbed CO₂ is referred to as “rich solution”) 1004 to release CO₂ and regenerates the CO₂ absorbent 1002. In this unit, the CO₂ absorbent having CO₂ reduced and being regenerated in the regenerator 1005 (hereinafter, the CO₂ absorbent having CO₂ reduced and being regenerated in the regenerator is referred to as “lean solution”) 1006 is reused as the CO₂ absorbent 1002 in the absorber 1003.

For example, the CO₂-containing flue gas 1001A that is the flue gas emitted from an industrial facility, such as a boiler or a gas turbine, is supplied to the absorber 1003 via a gas introducing line 1007. In the absorber 1003, the flue gas 1001A is brought into counter-current contact with the CO₂ absorbent 1002 that is alkanolamine based, for example, in a CO₂ absorbing unit 1010, and CO₂ contained in the CO₂-containing flue gas 1001A is absorbed by the CO₂ absorbent 1002 by way of a chemical reaction (R—NH₂+H₂O+CO₂(R—NH₃HCO₃). The mist of the CO₂ absorbent 1002 accompanying the CO₂-reduced flue gas 1001B is then collected by way of a CO₂ absorbing unit demister 1011, and the CO₂ absorbent 1002 is deposited in a bottom 1012 of the absorber 1003.

The CO₂-reduced flue gas 1001B is then washed by being brought into gas-liquid contact with water supplied from the top of the washing unit 1013 in the washing unit 1013. At this time, water supplied from a water tank or water supplied via a washing water supply line 1014, both of which are not illustrated, is used as the water for washing, and is supplied after being cooled in a heat exchanger 1015. The mist accompanying the CO₂-reduced flue gas 1001B is removed and collected at a washing unit demister 1016, and CO₂-reduced flue gas 1001C having mist removed is released out of the system from a top 1017 of the absorber 1003.

The pressure of the rich solution 1004 accumulated at the bottom 1012 of the absorber 1003 is increased through a rich solution supply line 1018 via a rich solvent pump 1019, heated in a rich/lean solution heat exchanger 1020, and supplied to the regenerator 1005.

The rich solution 1004 released into the regenerator 1005 is heated by steam 1022 from a regenerating heater 1021, releasing a major portion of the CO₂ and reducing CO₂ concentration to a low level, and the CO₂ absorbent 1002 is regenerated as the lean solution 1006. The released CO₂ passes through a recovering unit 1023 and a concentrating unit 1024; and CO₂ gas 1025 accompanying water vapor is released the exterior, and after the water is condensed and separated in a condenser 1026 and a separation drum 1027, CO₂ gas 1028 is released out of the system. The water separated in the separation drum 1027 is supplied to the regenerator 1005 or the absorber 1003 via circulating water supply lines 1031-1 and 1031-2 as circulating water 1030.

The lean solution 1006 is cooled in the rich/lean solution heat exchanger 1020, its pressure is raised by the lean solvent pump 1032, is further cooled in the lean solvent heat exchanger 1033, and then is supplied into the absorber 1003 as the CO₂ absorbent 1002.

In a reclaimer 1040, the lean solution 1006 is extracted through a lean solution supply line 1041, and retrograded substances such as salt remaining in the lean solution 1006 are extracted by way of a line 1042 and allowed to react with a basic sodium compound 1043, after which they are discharged as sludge 1044.

In FIG. 10, the CO₂ recovery apparatus 1000A may be added afterward so as to recover CO₂ emitted from an existing flue gas source, or may be installed together with a newly established flue gas source.

FIGS. 11 and 12 are schematics of other conventional CO₂ recovery apparatuses in which a process of supplying CO₂ absorbent 1002 is improved. As illustrated in FIG. 11, in another conventional CO₂ recovery apparatus 1000B, semi-lean solution (hereinafter, the CO₂ absorbent that has released a portion or a major portion of CO₂ in the regenerator is referred to as “semi-lean solution”) 1051 is extracted at the middle portion of the regenerator 1005 via a semi-lean solution extracting line 1050. The extracted semi-lean solution 1051 is cooled in a rich/semi-lean solution heat exchanger 1052 and a heat exchanger 1053, and then is supplied into the upper side of a lower CO₂ absorbing unit 1010-L so as to recover CO₂. The CO₂ absorbent 1002 extracted from the lower side of an upper CO₂ absorbing unit 1010-U is also supplied to the upper side of the lower CO₂ absorbing unit 1010-L in the absorber 1003 together with the semi-lean solution 1051 so as to recover CO₂ (Patent Document 2).

Furthermore, in another conventional CO₂ recovery apparatus 1000C, as illustrated in FIG. 12, the CO₂ absorbent 1002-1 and 1002-2, extracted at the middle portion of the CO₂ absorbing unit 1010 included in the absorber 1003 via CO₂ absorbent extracting lines 1060-1 and 1060-2, respectively, is cooled in heat exchangers 1061-1 and 1061-2, respectively, and then supplied into the absorber 1003 again so as to recover CO₂ (Patent Document 3).

In this manner, in operations recovering CO₂ by way of the chemical absorption technique, such as those in the conventional CO₂ recovery apparatuses 1000A to 1000C, the aqueous amine solution that is the CO₂ absorbent 1002 is separated from CO₂ by way of high-temperature steam, for example. The process of supplying the CO₂ absorbent 1002 is improved, whereby the CO₂ recovery efficiency is improved and, the steam (energy) consumption is decreased.

-   [Patent Document 1] Japanese Patent Laid-open No. H3-193116 -   [Patent Document 2] Specification in U.S. Pat. No. 6,800,120 -   [Patent Document 3] Japanese Patent No. 3416443

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In the method for absorption-reducing and recovering CO₂ from flue gas containing CO₂, such as flue gas, using the CO₂ absorbent 1002 and the processes described above, the operation cost needs to be reduced as much as possible because these processes are additionally provided in a combustion facility. Because the regenerating process requires a particularly large amount of thermal energy among the processes described above, it is necessary to reduce the energy of steam, thereby achieving the maximum energy savings in the process.

Furthermore, if the conventional CO₂ recovery apparatuses 1000A to 1000C become large in size and the amount of recovered CO₂ reaches equal to or more than 1000 tons per day, for example, a large amount of thermal energy is consumed in the regenerating process. Therefore, the energy of steam needs to be reduced, thereby achieving energy savings.

In consideration of the issues described above, an object of the present invention is to provide a CO₂ recovery apparatus and a CO₂ recovery method with improved energy efficiency by allowing the CO₂ absorbent to absorb a greater amount of CO₂ so as to reduce the mass flow of the circulated CO₂ absorbent, and to reduce the amount of steam required in regenerating the CO₂ absorbent in the regenerator.

Means for Solving Problem

According to an aspect of the present invention, a CO₂ recovery apparatus includes: an absorber that brings flue gas containing CO₂ into counter-current contact with CO₂ absorbent to reduce CO₂; and a regenerator that regenerates rich solution that has absorbed CO₂ through heat exchange, in which lean solution having CO₂ reduced in the regenerator is reused in the absorber. The absorber includes at least one CO₂ absorbing unit that recovers CO₂ in the flue gas containing CO₂, and the CO₂ absorbent that has absorbed CO₂ is extracted from a rich side of the CO₂ absorbing unit to exterior, cooled, then supplied to a position nearer to a lean side of the absorber with respect to the position at which the CO₂ absorbent is extracted, and reused as the CO₂ absorbent to recover CO₂ so as to increase a CO₂ concentration in the CO₂ absorbent.

Advantageously, in the CO₂ recovery apparatus, the absorber includes two such CO₂ absorbing units that are an upper-stage CO₂ absorbing unit and a lower-stage CO₂ absorbing unit. The CO₂ recovery apparatus further includes: an absorbent circulating line through which CO₂ absorbent extracted from a rich side of the lower-stage CO₂ absorbing unit is supplied to a lean side of the lower-stage CO₂ absorbing unit; an absorbent extracting line through which CO₂ absorbent extracted from a rich side of the upper-stage CO₂ absorbing unit is merged with CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit; and a heat exchanger that is arranged in the absorbent circulating line, and cools the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted via the absorbent extracting line.

Advantageously, in the CO₂ recovery apparatus, the absorber includes three such CO₂ absorbing units that are an upper-stage CO₂ absorbing unit, a middle-stage CO₂ absorbing unit, and a lower-stage CO₂ absorbing unit. The CO₂ recovery apparatus further includes: an absorbent circulating line through which CO₂ absorbent extracted from a rich side of the middle-stage CO₂ absorbing unit is supplied to a lean side of the middle-stage CO₂ absorbing unit; an absorbent extracting line through which CO₂ absorbent extracted from a rich side of the upper-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit; and a heat exchanger that is arranged in the absorbent circulating line, and cools the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the rich side of the upper-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted via the absorbent extracting line.

Advantageously, the CO₂ recovery apparatus, further includes a circulating absorbent extracting line through which a part of the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit via the absorbent circulating line is supplied to a lean side of the lower-stage CO₂ absorbing unit.

Advantageously, in the CO₂ recovery apparatus, the absorber includes three such CO₂ absorbing units that are an upper-stage CO₂ absorbing unit, a middle-stage CO₂ absorbing unit, and a lower-stage CO₂ absorbing unit. The CO₂ recovery apparatus further includes: an absorbent circulating line through which CO₂ absorbent extracted from a rich side of the lower-stage CO₂ absorbing unit is supplied to a lean side of the middle-stage CO₂ absorbing unit; an absorbent extracting line through which CO₂ absorbent extracted from a rich side of the upper-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit; and a heat exchanger that is arranged in the absorbent circulating line, and cools the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted via the absorbent extracting line.

Advantageously, in the CO₂ recovery apparatus, CO₂ absorbent extracted from a rich side of the middle-stage CO₂ absorbing unit is supplied to a lean side of the lower-stage CO₂ absorbing unit.

Advantageously, the CO₂ recovery apparatus, further includes a heat exchanger that cools the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit.

Advantageously, in the CO₂ recovery apparatus, an end of the absorbent extracting line is connected to a rich solution supply line through which the rich solution is supplied to the regenerator, and the rich solution is extracted through the rich solution supply line.

Advantageously, in the CO₂ recovery apparatus, a mass flow ratio of a mass flow of the CO₂ absorbent supplied to the absorbent circulating line with respect to a mass flow of the rich solution supplied to the rich solution supply line through which the rich solution is supplied to the regenerator is between 0.1 to 1.0 and 6.0 to 1.0.

Advantageously, in the CO₂ recovery apparatus, a temperature of the CO₂ absorbent extracted from the CO₂ absorbing unit after being cooled is 30 to 50 degrees Celsius.

According to another aspect of the present invention, a CO₂ recovery method, includes after bringing flue gas containing CO₂ into counter-current contact with CO₂ absorbent in a CO₂ absorbing unit included in an absorber to reduce CO₂, regenerating rich solution that has absorbed CO₂ in a regenerator, then reusing regenerated lean solution having CO₂ reduced in the absorber. The CO₂ recovery method includes: extracting the CO₂ absorbent that has absorbed CO₂ from a rich side of the CO₂ absorbing unit to exterior, cooling the same, supplying the same to a position nearer to a lean side of the absorber with respect to the position at which the CO₂ absorbent is extracted, and using the same as the CO₂ absorbent to recover CO₂, thereby increasing a CO₂ concentration in the CO₂ absorbent.

Advantageously, the CO₂ recovery method further includes: extracting the CO₂ absorbent from a rich side of a lower-stage CO₂ absorbing unit in the absorber including two such CO₂ absorbing units that are an upper-stage CO₂ absorbing unit and the lower-stage CO₂ absorbing unit; merging CO₂ absorbent extracted from a rich side of the upper-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit; cooling the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the rich side of the upper-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit; and supplying the CO₂ absorbent thus merged to a lean side of the lower-stage CO₂ absorbing unit so as to recover CO₂ contained in the flue gas containing CO₂.

Advantageously, the CO₂ recovery method further includes: extracting CO₂ absorbent from a rich side of a middle-stage CO₂ absorbing unit in the absorber including three such CO₂ absorbing units that are an upper-stage CO₂ absorbing unit, the middle-stage CO₂ absorbing unit, and a lower-stage CO₂ absorbing unit; merging CO₂ absorbent extracted from a rich side of the upper-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit; cooling the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted from the rich side of the upper-stage CO₂ absorbing unit; and supplying the CO₂ absorbent thus merged to a lean side of the middle-stage CO₂ absorbing unit so as to recover CO₂ contained in the gas containing CO₂.

Advantageously, the CO₂ recovery method further includes supplying a part of the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit to a lean side of the lower-stage CO₂ absorbing unit.

Advantageously, the CO₂ recovery method further includes: extracting the CO₂ absorbent from a rich side of a lower-stage CO₂ absorbing unit in the absorber including three such CO₂ absorbing units that are an upper-stage CO₂ absorbing unit, a middle-stage CO₂ absorbing unit, and the lower-stage CO₂ absorbing unit; merging CO₂ absorbent extracted from a rich side of the upper-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit; cooling the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the rich side of the upper-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit; and supplying the CO₂ absorbent thus merged to a lean side of the middle-stage CO₂ absorbing unit so as to recover CO₂ contained in the gas containing CO₂.

Advantageously, the CO₂ recovery method further includes supplying CO₂ absorbent extracted from a rich side of the middle-stage CO₂ absorbing unit to a lean side of the lower-stage CO₂ absorbing unit.

Advantageously, the CO₂ recovery method further includes cooling the CO₂ absorbent extracted from the rich side of the middle-stage CO₂ absorbing unit and thereafter supplying the CO₂ absorbent to the lean side of the lower-stage CO₂ absorbing unit.

Advantageously, the CO₂ recovery method further includes extracting rich solution from a rich solution supply line through which the rich solution is supplied to the regenerator; and circulating the rich solution extracted through the rich solution supply line to the rich side of any one of the upper-stage CO₂ absorbing unit, the middle-stage CO₂ absorbing unit, and the lower-stage CO₂ absorbing unit.

Advantageously, the CO₂ recovery method further includes extracting the rich solution, wherein a dividing ratio between a circulation mass flow of the CO₂ absorbent extracted from the rich side of the lower-stage CO₂ absorbing unit to any one of the upper-stage CO₂ absorbing unit, the middle-stage CO₂ absorbing unit, and the lower-stage CO₂ absorbing unit and a mass flow of the rich solution supplied to the regenerator is between 0.1 to 1.0 and 6.0 to 1.0.

Advantageously, in the CO₂ recovery method, a temperature of the CO₂ absorbent extracted from the CO₂ absorbing unit after being cooled is 30 to 50 degrees Celsius.

Advantageously, in the CO₂ recovery method, a mass flow ratio between a mass flow of the rich solution supplied to the regenerator and an amount of absorbent circulation is optimally controlled depending on a gas temperature of flue gas introduced to the absorber so as to reduce operation costs.

Advantageously, in the CO₂ recovery method, when the CO₂ absorbent has a decreased concentration, an amount of circulation in an entire system is increased to maintain a CO₂ absorption capability.

Effect of the Invention

According to the present invention, by extracting the CO₂ absorbent that has absorbed CO₂ from the rich side of the CO₂ absorbing unit in the absorber, cooling the same, and supplying the same to the lean side of the absorber with respect to the position at which the CO₂ absorbent is extracted, the amount of CO₂ recovered per unit amount of the CO₂ absorbent can be increased, and the CO₂ concentration in the rich solution at the bottom of the absorber can also be increased. Therefore, the rich solution with a higher CO₂ concentration can be supplied to the regenerator, and the mass flow of the CO₂ absorbent circulated between the absorber and the regenerator can be reduced. In this manner, the steam required in regenerating the CO₂ absorbent and the load of the condenser can be reduced, the thermal energy can be reduced, and energy savings can be achieved.

As a result, further energy savings can be achieved in the process of recovering the CO₂ absorbent, even in a large CO₂ recovery apparatus that recovers a greater amount of CO₂, because the thermal energy can be reduced by supplying the rich solution with a higher CO₂ concentration to the regenerator to reduce the amount of steam required in the process of regeneration.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a CO₂ recovery apparatus according to a first embodiment of the present invention.

FIG. 2 is a schematic of an alternative structure of the CO₂ recovery apparatus according to the first embodiment.

FIG. 3A is a schematic of a CO₂ recovery apparatus according to a second embodiment of the present invention.

FIG. 3B is a schematic of the CO₂ recovery apparatus according to the second embodiment.

FIG. 4 is a schematic of a relationship between a mass flow of circulating absorbent with respect to a mass flow of rich solution, and the reduction rate of energy required in regenerating the absorbent.

FIG. 5 is a schematic of the mass flow ratio of the mass flow of the circulating absorbent with respect to the mass flow of the rich solution depending on gas temperature, and the rate of reduction in a CO₂ recovery operation cost.

FIG. 6 is a schematic for explaining the difference in the amount of absorbent circulation and a CO₂ loading rate between the CO₂ recovery apparatus according to the present embodiment and a conventional CO₂ recovery apparatus.

FIG. 7 is a schematic of an alternative structure of the CO₂ recovery apparatus according to the second embodiment of the present invention.

FIG. 8 is a schematic of a CO₂ recovery apparatus according to a third embodiment of the present invention.

FIG. 9 is a schematic of a CO₂ recovery apparatus according to a fourth embodiment of the present invention.

FIG. 10 is a schematic of an example of a structure of a conventional CO₂ recovery apparatus.

FIG. 11 is a schematic of an alternative structure of a conventional CO₂ recovery apparatus.

FIG. 12 is a schematic of another alternative structure of a conventional CO₂ recovery apparatus.

EXPLANATIONS OF LETTERS OR NUMERALS

-   10A to 10F CO₂ recovery apparatus -   11 circulating absorbent -   11 a merged circulating absorbent -   12A to 12E absorbent circulating line -   13 heat exchanger -   14 pump -   15A to 15C absorbent extracting line -   16 circulating absorbent extracting line -   17 heat exchanger -   1001A CO₂-containing flue gas -   1001B, 1001C CO₂-reduced flue gas -   1002 CO₂ absorbent -   1002 a, 1002 b extracted CO₂ absorbent -   1003 absorber -   1004 rich solution -   1005 regenerator -   1006 lean solution (regenerated solution) -   1007 gas introducing line -   1010 CO₂ absorbing unit -   1010-U upper-stage CO₂ absorbing unit -   1010-M middle-stage CO₂ absorbing unit -   1010-L lower-stage CO₂ absorbing unit -   1011 CO₂ absorbing unit demister -   1012 bottom -   1013 washing unit -   1014 washing water supply line -   1015 heat exchanger -   1016 washing unit demister -   1017 top -   1018 rich solution supply line -   1019 rich solvent pump -   1020 rich/lean solution heat exchanger -   1021 regenerating heater -   1022 steam -   1023 recovering unit -   1024 concentrating unit -   1025 CO₂ gas -   1026 condenser -   1027 separation drum -   1028 CO₂ gas -   1030 circulating water -   1031-1, 1031-2 circulating water supply line -   1032 lean solvent pump -   1033 lean solvent heat exchanger -   1040 reclaimer -   1041 lean solution supply line -   1042 lean solution extracting line -   1043 basic sodium compound -   1044 sludge -   1050 semi-lean solution extracting line -   1051 semi-lean solution -   1052 rich/semi-lean solution heat exchanger -   1053 heat exchanger -   1060-1, 1060-2 CO₂ absorbent extracting line -   1061-1, 1061-2 heat exchanger

BEST MODE(S) FOR CARRYING OUT THE INVENTION

The present invention will now be explained in detail with reference to some drawings. Embodiments of the present invention disclosed therein are not intended to limit the scope of the present invention in any way. Furthermore, elements disclosed in the embodiments include those that can be easily thought of by those skilled in the art, and those that are the substantially identical.

First Embodiment

A CO₂ recovery apparatus according to a first embodiment of the present invention will now be explained with reference to a drawing.

FIG. 1 is a schematic of the CO₂ recovery apparatus according to the first embodiment. The same elements as those included in the CO₂ recovery apparatus 1000A illustrated in FIG. 10 are given with the same reference numerals, and redundant explanations thereof are omitted hereunder.

As illustrated in FIG. 1, a CO₂ recovery apparatus 10A according to the first embodiment includes: the absorber 1003 in which the CO₂-containing flue gas 1001A is brought into counter-current contact with the CO₂ absorbent 1002 to reduce CO₂, and the regenerator 1005 that regenerates the rich solution 1004 by allowing the rich solution 1004 that has absorbed CO₂ to exchange heat. The lean solution (regenerated solution) 1006 having CO₂ reduced in the regenerator 1005 is reused in the absorber 1003. The absorber 1003 further includes the CO₂ absorbing unit 1010 that recovers CO₂ in the CO₂-containing flue gas 1001A, and the CO₂ absorbent 1002 that has absorbed CO₂ is extracted to the exterior from the rich side of the CO₂ absorbing unit 1010, cooled, and then supplied to the lean side of the absorber 1003 from the position at which the CO₂ absorbent 1002 is extracted.

In the first embodiment, the CO₂ absorbent 1002 that has absorbed CO₂ and extracted from the lower side of the CO₂ absorbing unit 1010 is used as circulating absorbent 11.

In the present invention, the lean side refers to the side that is upstream of the CO₂ absorbent 1002 supplied to the CO₂ absorbing unit 1010, and, in the first embodiment, refers to the upper side of the CO₂ absorbing unit 1010.

The rich side refers to the side that is upstream of the CO₂-containing flue gas 1001A supplied to the CO2 absorbing unit 1010, and in this embodiment, refers to the lower side of the CO₂ absorbing unit 1010.

The CO₂ recovery apparatus 10A according to the first embodiment includes an absorbent circulating line 12A for extracting the CO₂ absorbent 1002 that has absorbed CO₂ from the lower side of the CO₂ absorbing unit 1010 as the circulating absorbent 11, and for supplying the circulating absorbent 11 to the lean side of the CO₂ absorbing unit 1010 with respect the position at which the CO₂ absorbent 1002 is extracted in the absorber 1003. By returning the circulating absorbent 11 containing CO₂ as the circulating absorbent 11 to the CO₂ absorbing unit 1010 as the CO₂ absorbent 1002, because the CO₂ absorbent 1002 has not reached a saturation level under which CO₂ contained in the CO₂-containing flue gas 1001A can be absorbed, CO₂ contained in the CO₂-containing flue gas 1001A can be recovered using the CO₂ absorbent 1002 that is still capable of absorbing CO₂, and the CO₂ concentration in the CO₂ absorbent 1002 can be increased.

In addition, in the CO₂ recovery apparatus 10A according to the first embodiment, the absorbent circulating line 12A includes a heat exchanger 13 for cooling the circulating absorbent 11 that is extracted from the CO₂ absorbing unit 1010 via the absorbent circulating line 12A. When the temperature of the rich solution 1004 rises, the balanced partial pressure increases at the bottom 1012 of the absorber 1003 because the rich solution 1004 contains a large amount of CO₂, reducing the capacity of the rich solution 1004 to absorb CO₂ contained in the CO₂-containing flue gas 1001A. Therefore, by cooling the circulating absorbent 11 in the heat exchanger 13, and supplying the cooled circulating absorbent 11 to the CO₂ absorbing unit 1010, the balanced partial pressure of the CO₂ absorbent 1002 can be reduced, so that the CO₂ absorbent 1002 can absorb more CO₂.

The temperature of the circulating absorbent 11 after being cooled in the heat exchanger 13 is preferably 30 to 50 degrees Celsius. This is in order to reduce the balanced partial pressure and to improve the CO₂ recovery efficiency of the CO₂ absorbent 1002.

The circulating absorbent 11 is extracted by way of a pump 14 arranged in the absorbent circulating line 12A, and the mass flow of the circulating absorbent 11 that is extracted into the absorbent circulating line 12A is adjusted with, for example, a valve V1. The mass flow of the rich solution 1004 that is supplied to the rich solution supply line 1018 is adjusted with, for example, a valve V2. Therefore, the dividing ratio of the mass flow of the rich solution 1004 divided between the absorbent circulating line 12A and to the rich solution supply line 1018 is adjusted by way of the valves V1 and V2.

Furthermore, while in the CO₂ recovery apparatus 10A according to the first embodiment, the rich solution 1004 is separately extracted via the absorbent circulating line 12A and the rich solution supply line 1018 at the bottom 1012 of the absorber 1003 in the CO₂ recovery apparatus 10A, the present invention is not limited thereto.

For example, as illustrated in FIG. 2, in a CO₂ recovery apparatus 10B, an absorbent circulating line 12B may split from the rich solution supply line 1018 that supplies the rich solution 1004 to the regenerator 1005, so as to extract the rich solution 1004 from the rich solution supply line 1018, and the extracted rich solution 1004 may be supplied to the CO₂ absorbing unit 1010 as the circulating absorbent 11. Furthermore, while in the CO₂ recovery apparatus 10B illustrated in FIG. 2, the rich solution 1004 is extracted through the rich solution supply line 1018 at a position near the bottom 1012 of the absorber 1003, the present invention is not especially limited thereto.

As described above, according to the first embodiment, the CO₂ absorbent 1002 that has absorbed CO₂ is extracted as the circulating absorbent 11 to the exterior from the lower side of the CO₂ absorbing unit 1010 in the absorber 1003, cooled, and then supplied to the lean side of the CO₂ absorbing unit 1010 in the absorber 1003. In this manner, the CO₂ recovery efficiency in the absorber 1003 is improved, and the amount of CO₂ recovered per unit amount of the CO₂ absorbent can be increased. As a result, the CO₂ concentration in the rich solution 1004 at the bottom 1012 of the absorber 1003 can be increased, and therefore, the rich solution 1004 with a higher CO₂ concentration can be supplied to the regenerator 1005, thus reducing the mass flow of the CO₂ absorbent 1002 circulated between the absorber 1003 and the regenerator 1005. In this manner, the mass flow of the rich solution 1004 supplied to the regenerator 1005 can be reduced. Therefore, the amount of supplied steam used in regenerating the CO₂ absorbent 1002 in the regenerator 1005 can be reduced, and thermal energy consumption can be reduced. In this manner, the energy efficiency can be improved.

As a result, even in a large CO₂ recovery apparatus that recovers 1000 tons of CO₂ per day, for example, the thermal energy can be reduced by increasing the CO₂ concentration in the rich solution 1004 supplied to the regenerator 1005 to reduce the amount of steam required in the regenerator 1005. Thus, further energy savings can be achieved in the process of regenerating the CO₂ absorbent 1002.

Furthermore, while in the CO₂ recovery apparatus 10A according to the first embodiment, the CO₂ absorbent 1002 that has absorbed CO₂ is extracted from the lower side of the CO₂ absorbing unit 1010 as the circulating absorbent 11, the present invention is not limited thereto, and the CO₂ absorbent 1002 that has absorbed CO₂ can be extracted to the exterior from the rich side of the CO₂ absorbing unit 1010.

The type of the heat exchanger 13 used in the first embodiment is not especially limited, and any known heat exchanger such as a plate heat exchanger, or a shell and tube heat exchanger may be used.

Furthermore, the CO₂ absorbent 1002 that can be used in the present invention is not especially limited, and examples thereof include alkanolamines, or hindered amines having an alcoholic hydroxyl group. Examples of such alkanolaminse include monoethanol amine, dinoethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine, and diglycolamin; however, usually, monoethanolamine (MEA) is preferably used. Examples of hindered amines having an alcoholic hydroxyl group include 2-amino-2-methyl-1-propanol (AMP), 2-(ethylamino)-ethanol (EAE), 2-(methylamino)-ethanol (MAE), and 2-(diethylamino)ethanol (DEAF).

The CO₂ absorbent 1002 may be one of these compounds, or a mixture of two or more of these compounds. A basic amine compound is usually used as an aqueous solution with a concentration of 10 to 70 percent by weight. Furthermore, a carbon dioxide absorption promoter or a corrosion inhibitor can be added to the CO₂ absorbent 1002, and methanol, polyethyleneglycol, sulfolane, or similar compounds may be added thereto as other agents.

Second Embodiment

A CO₂ recovery apparatus according to a second embodiment of the present invention will now be explained with reference to FIGS. 3A and 3B.

FIGS. 3A and 3B are schematics of the CO₂ recovery apparatuses according to the second embodiment. The members that are similar to those according to the first embodiment are given with the same reference numerals, and redundant explanations thereof are omitted hereunder.

As illustrated in FIG. 3A, in a CO₂ recovery apparatus 10C according to the second embodiment, the absorber 1003 includes two CO₂ absorbing unit 1010 that are the upper-stage CO₂ absorbing unit 1010-U and the lower-stage CO₂ absorbing unit 1010-L, respectively. The absorber 1003 further includes an absorbent circulating line 12C for extracting the CO₂ absorbent 1002 from the lower side of the lower-stage CO₂ absorbing unit 1010-L as the circulating absorbent 11, and supplying the extracted circulating absorbent 11 to the upper side of the lower-stage CO₂ absorbing unit 1010-L; an absorbent extracting line 15A for merging the CO₂ absorbent 1002 extracted from the lower side of the upper-stage CO₂ absorbing unit 1010-U with the CO₂ absorbent 1002 extracted from the lower side of the lower-stage CO₂ absorbing unit 1010-L; and the heat exchanger 13 arranged in the absorbent circulating line 12C to cool the circulating absorbent 11 before the circulating absorbent 11 is merged to a CO₂ absorbent 1002 a extracted via the absorbent extracting line 15A.

In the CO₂ recovery apparatus 10C according to the second embodiment, the CO₂ absorbent 1002 is extracted from the lower side of the lower-stage CO₂ absorbing unit 1010-L₂ into the absorbent circulating line 12C as the circulating absorbent 11, and the CO₂ absorbent 1002 a extracted from the lower side of the upper-stage CO₂ absorbing unit 1010-U via the absorbent extracting line 15A is merged with the circulating absorbent 11. The CO₂ absorbent 1002 a supplied via the absorbent extracting line 15A and merged circulating absorbent 11 a are supplied to the upper side of the lower-stage CO₂ absorbing unit 1010-L. In this manner, because the CO₂ absorbent 1002 has not reached the saturation level under which CO₂ contained in the CO₂-containing flue gas 1001A can be absorbed, CO₂ contained in the CO₂-containing flue gas 1001A can be recovered using the CO₂ absorbent 1002 that is still capable of absorbing CO₂, and the CO₂ concentration in the CO₂ absorbent 1002 can be increased. In FIG. 3A, the reference numeral 19 denotes a pump inserted to the absorbent extracting line 15A.

Furthermore, in the CO₂ recovery apparatus 10C according to the second embodiment, the circulating absorbent 11 extracted from the lower-stage CO₂ absorbing unit 1010-L to the absorbent circulating line 12C via the absorbent circulating line 12C is cooled in the heat exchanger 13. By cooling the circulating absorbent 11 in the heat exchanger 13 and supplying the cooled circulating absorbent 11 to the lower-stage CO₂ absorbing unit 1010-L, the balanced partial pressure of the CO₂ absorbent 1002 can be reduced. Therefore, more CO₂ can be absorbed by the CO₂ absorbent 1002.

Furthermore, while in the CO₂ recovery apparatus 10C according to the second embodiment, the heat exchanger 13 is arranged upstream to the position at which the absorbent circulating line 12C is connected to the absorbent extracting line 15A, the present invention is not limited thereto. For example, as another embodiment in FIG. 3B illustrates, the heat exchanger 13 may be arranged downstream to the position at which the absorbent circulating line 12C is connected to the absorbent extracting line 15A.

By allowing the absorbent 1002 a extracted from the upper-stage CO₂ absorbing unit 1010-U to fall into the absorbent circulating line 12C near the lower area of the absorber 1003 and to merge, the absorbent 1002 a is introduced to the absorbent circulating line 12C under its own weight upon falling, and thus, it becomes unnecessary to force the absorbent 1002 a thereto by way of a pump 19 such as the one illustrated in FIG. 3A, thereby allowing the structure of the unit to be simplified.

Furthermore, in the CO₂ recovery apparatus 10C according to the second embodiment, the mass flow ratio of the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 is set preferably between 0.1 to 1.0 and 6.0 to 1.0, and more preferably between 1.0 to 1.0 and 4.0 to 1.0. This is because the energy required in regenerating the CO₂ absorbent 1002 can be reduced by increasing the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C. This is also because, if the ratio of the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 is equal to or higher than 4.0 to 1.0, the energy required in regenerating the CO₂ absorbent 1002 can hardly be reduced. This is also because, if the mass flow ratio of the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 increases to equal to or higher than 6.0 to 1.0, a phenomenon referred to as flooding occurs, in which the level of the CO₂ absorbent 1002 rises in the absorber 1003.

FIG. 4 is a schematic of a relationship between the mass flow ratio of the mass flow of the circulating absorbent supplied to the absorbent circulating line with respect to the mass flow of the rich solution supplied to the rich solution supply line, and a reduction rate of the energy required for absorbent regeneration.

As illustrated in FIG. 4, the increase in the mass flow ratio of the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 has lead to the increase in reduction rate in the energy required in absorbent regeneration. Furthermore, when the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the ratio of the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 was set to approximately 1.0, the reduction rate of the energy required in regenerating the CO₂ absorbent 1002 was approximately 5.0 percent. When the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 was set to approximately 4.0, the reduction rate of the energy required in regenerating the CO₂ absorbent 1002 was approximately 10.5 percent. Furthermore, the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 was set to approximately between 4.0 and 6.0, the reduction rate of the energy required in regenerating the CO₂ absorbent 1002 was approximately 11 percent, and did not increase very much.

Therefore, the mass flow ratio of the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 is preferably between 0.1 and 6.0, and more preferably between 1.0 and 4.0.

Upon introducing the flue gas to the absorber 1003, a cooler (not illustrated) installed before the absorber 1003 is used to cool the flue gas 1001A introduced thereto. Depending on the introduction temperature to the absorber 1003, the mass flow ratio of the circulated amount of the circulating absorbent 11 (L2) with respect to the mass flow of the rich solution 1004 supplied to the regenerator 1005 (L1) may be optimally controlled (L2/L1=1.5 to 4.5) so that a reduction in the operation cost is achieved.

FIG. 5 is a schematic of a gas-temperature dependent relationship between the mass flow ratio (L2/L1) of the mass flow of the circulating absorbent 11 in the absorbent circulating line 12A with respect to the mass flow (L1) of the rich solution 1004 supplied to the regenerator, and the reduction rate of the CO₂ recovery operation cost depending on temperature.

The solid line in FIG. 5 represents a scenario in which the temperature of the gas introduced into the absorber 100 is low at 35 degrees Celsius, and the dotted line in FIG. 5 represents a scenario in which the temperature of the gas introduced into the absorber 100 is relatively high at 50 degrees Celsius.

As may be obvious from FIG. 5, the optimal mass flow ratio (L2/L1) of the circulated amount (L2) of the circulating absorbent 11 with respect to the mass flow (L1) of the rich solution 1004 supplied to the regenerator 1005 is between 1.5 to 4.0 at 35 degrees Celsius, and the optimal mass flow ratio (L2/L1) of the circulated amount (L2) of the circulating absorbent 11 with respect to the mass flow (L1) of the rich solution 1004 supplied to the regenerator 1005 is between approximately 3.0 to 4.5 at 50 degrees Celsius.

Therefore, by optimizing the amount of circulation depending on the temperature of the gas introduced to the absorber 1003, the running costs can further be reduced.

Furthermore, in a scenario where the concentration of the CO₂ absorbent declines as a result of extended operation, a predetermined amount of CO₂ can constantly be recovered by increasing the amount of the circulation in the entire system to maintain the CO₂ absorption capacity.

The temperature of the rich solution 1004 at the bottom 1012 of the absorber 1003, the amount of the circulation of the CO₂ absorbent 1002 circulated between the CO₂ absorber 1003 and the regenerator 1005, and a CO₂ loading rate of the absorbent 1002 recovering CO₂ with the use of the CO₂ recovery apparatus 10C according to the second embodiment will now be explained using FIG. 5.

FIG. 6 is a schematic for explaining the amount of the circulation of the CO₂ absorbent circulated between the absorber and the regenerator, and the CO₂ loading rate in each of the CO₂ recovery apparatus 10C according to the second embodiment and the conventional CO₂ recovery apparatus such as the one illustrated in FIG. 10.

Furthermore, in FIG. 6, a test using the CO₂ recovery apparatus 10C according to the second embodiment is denoted as a test example 1, and a test using the conventional CO₂ recovery apparatus 1000A such as the one illustrated in FIG. 10 is denoted as a comparative example 1.

Furthermore, in the test example 1, the mass flow ratio of the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 was set to approximately 4.0.

As illustrated in FIG. 6, the test example 1 using the CO₂ recovery apparatus 10C according to the second embodiment, such as the one illustrated in FIG. 3A, demonstrated that the temperature of the rich solution 1004 at the bottom 1012 of the absorber 1003 decreased by approximately 7 degrees Celsius, in comparison with the comparative example 1 using the conventional CO₂ recovery apparatus 1000A such as the one illustrated in FIG. 10.

Furthermore, assuming that the amount of the circulation of the CO₂ absorbent 1002 between the absorber 1003 and the regenerator 1005 in the comparative example 1 using the conventional CO₂ recovery apparatus 1000A, such as the one illustrated in FIG. 10, is 1.0 (reference value), the mass flow rate of the amount of circulation of the CO₂ absorbent 1002 between the absorber 1003 and the regenerator 1005 in the test example 1 using the CO₂ recovery apparatus 10C according to the second embodiment, such as the one illustrated in FIG. 3, with respect to that in the comparative example 1 using the conventional CO₂ recovery apparatus 1000A, such as the one illustrated in FIG. 10, was 0.91.

The amount of recovered CO₂ can be obtained from the value of the amount of the circulation of the CO₂ absorbent 1002 between the absorber 1003 and the regenerator 1005, the value of the CO₂ concentration in the rich solution 1004, and the value of the CO₂ concentration in the lean solution 1006, as indicated by formula (1): ΔCO₂ =L×(CR−CL)  (1)

Where, the amount of recovered CO₂ is denoted as ΔCO₂; the amount of the circulation of the CO₂ absorbent between the absorber and the regenerator is denoted as L; the CO₂ concentration in the rich solution is denoted as CR; and the CO₂ concentration in the lean solution is denoted as CL.

In other words, assuming that a predetermined amount of CO₂ is to be recovered, because the lean solution 1006 is the solution regenerated as the CO₂ absorbent 1002 and the CO₂ concentration CL in the lean solution 1006 is constant, if the CO₂ concentration CR in the rich solution 1004 increases, the difference between the CO₂ concentration in the rich solution 1004 and that in the lean solution 1006 “CR−CL” increases correspondingly based on formula (1). Therefore, the amount of the circulation L of the CO₂ absorbent 1002 circulated between the absorber 1003 and the regenerator 1005 required for recovering a predetermined amount of CO₂ can be reduced.

Therefore, when the mass flow ratio of the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 is approximately 4.0, because the mass flow ratio of the amount of the circulation of the CO₂ absorbent 1002 between the absorber 1003 and the regenerator 1005 in the test example 1, in which the CO₂ recovery apparatus 10C according to the second embodiment such as the one illustrated in FIG. 3 is used, with respect to that in the comparative example 1, in which the conventional CO₂ recovery apparatus 1000A such as the one illustrated in FIG. 10 is used, is 0.91 percent, the CO₂ recovery apparatus 10C according to the second embodiment, such as the one illustrated in FIG. 3A, achieves a reduction of 10 percent in the amount of the circulation of the CO₂ absorbent 1002 between the absorber 1003 and the regenerator 1005, in comparison with a scenario in which the conventional CO₂ recovery apparatus 1000A, such as the one illustrated in FIG. 10, is used.

Furthermore, assuming that the CO₂ loading rate of the CO₂ absorbent 1002 recovering CO₂ in the conventional CO₂ recovery apparatus 1000A, such as the one illustrated in FIG. 10, is 1.0 (reference value), the CO₂ loading rate in the CO₂ recovery apparatus 10C according to the second embodiment was 1.08, with respect to that in the conventional CO₂ recovery apparatus 1000A.

Therefore, when the mass flow ratio of the mass flow of the circulating absorbent 11 supplied to the absorbent circulating line 12C with respect to the mass flow of the rich solution 1004 supplied to the rich solution supply line 1018 is approximately 4.0, the CO₂ recovery apparatus 10C according to the second embodiment can increase the CO₂ loading ratio, with respect to that achieved by the conventional CO₂ recovery apparatus 1000A such as the one illustrated in FIG. 10. Therefore, by using the CO₂ recovery apparatus 10C according to the second embodiment, such as the one illustrated in FIG. 3A, the amount of CO₂ recovered by the rich solution 1004 can be increased, and the CO₂ concentration in the rich solution 1004 can also be increased, in comparison with a scenario in which the conventional CO₂ recovery apparatus 1000A, such as the one illustrated in FIG. 10, is used.

Furthermore, in these tests, the temperature of the rich solution 1004 at the bottom 1012 of the absorber 1003 was set to approximately 44 degrees Celsius. However, the CO₂ loading rate in the CO₂ recovery apparatus 10C according to the second embodiment can be increased to, for example, approximately 1.2 with respect to that achieved in the conventional CO₂ recovery apparatus 1000A, by decreasing the temperature of the rich solution 1004 at the bottom 1012 of the absorber 1003.

Furthermore, while in the CO₂ recovery apparatus 10C according to the second embodiment, each of the CO₂ absorbing units 1010, the upper-stage CO₂ absorbing unit 1010-U and the lower-stage CO₂ absorbing unit 1010-L included in the absorber 1003 are the same in size, the present invention is not limited thereto, and the lower-stage CO₂ absorbing unit 1010-L and the upper-stage CO₂ absorbing unit 1010-U may be different in size.

For example, as illustrated in FIG. 7, the capacity of the upper-stage CO₂ absorbing unit 1010-U may be double that of the lower-stage CO₂ absorbing unit 1010-L, so that CO₂ contained in the CO₂-containing flue gas 1001A is recovered effectively in the upper-stage CO₂ absorbing unit 1010-U into which the lean solution 1006 is supplied directly.

As described above, according to the second embodiment, because the CO₂ absorbent 1002 is extracted from the lower side of the lower-stage CO₂ absorbing unit 1010-L in the absorber 1003 as the circulating absorbent 11, cooled, and then supplied to the upper side of the lower-stage CO₂ absorbing unit 1010-L, the CO₂ recovery efficiency of the absorber 1003 can be improved, and the amount of CO₂ recovered per unit amount of the CO₂ absorbent can be increased. As a result, because the CO₂ concentration in the rich solution 1004 at the bottom 1012 of the absorber 1003 can be increased, the rich solution 1004 with a higher CO₂ concentration can be supplied to the regenerator 1005, and the mass flow of the CO₂ absorbent 1002 circulated between the absorber 1003 and the regenerator 1005 can be reduced. In this manner, because the mass flow of the rich solution 1004 supplied to the regenerator 1005 can be reduced, the amount of the supplied steam that is used in regenerating the CO₂ absorbent 1002 in the regenerator 1005 can be reduced, to further reduce thermal energy consumption and to improve the energy efficiency.

Third Embodiment

A CO₂ recovery apparatus according to a third embodiment of the present invention will now be explained with reference to FIG. 8.

FIG. 8 is a schematic of the CO₂ recovery apparatus according to the third embodiment. The members that are similar to those according to the first embodiment are given with the same reference numerals, and redundant explanations thereof are omitted hereunder.

As illustrated in FIG. 8, in a CO₂ recovery apparatus 10E according to the third embodiment, the absorber 1003 includes: three CO₂ absorbing units 1010 that are the upper-stage CO₂ absorbing unit 1010-U, a middle-stage CO₂ absorbing unit 1010-M, and the lower-stage CO₂ absorbing unit 1010-L; an absorbent circulating line 12D for extracting the CO₂ absorbent 1002 from the lower side of the middle-stage CO₂ absorbing unit 1010-M as the circulating absorbent 11, and for supplying the extracted circulating absorbent 11 to the upper side of the middle-stage CO₂ absorbing unit 1010-M; an absorbent extracting line 15B for merging the CO₂ absorbent 1002 a extracted from the lower side of the upper-stage CO₂ absorbing unit 1010-U with the circulating absorbent 11; and the heat exchanger 13 arranged in the absorbent circulating line 12D to cool the circulating absorbent 11 before the circulating absorbent 11 is merged with the CO₂ absorbent 1002 a extracted via the absorbent extracting line 15B.

In the CO₂ recovery apparatus 10E according to the third embodiment, the CO₂ absorbent 1002 is extracted from the lower side of the middle-stage CO₂ absorbing unit 1010-M as the circulating absorbent 11 in the absorber 1003, and the CO₂ absorbent 1002 a extracted from the lower side of the upper-stage CO₂ absorbing unit 1010-U via absorbent extracting line 15B is merged with the circulating absorbent 11, and circulating absorbent 11 a that is the circulating absorbent 11 thus merged is supplied to the upper side of the middle-stage CO₂ absorbing unit 1010-M. Because the merged circulating absorbent 11 a has not reached the saturation level under which CO₂ contained in the CO₂-containing flue gas 1001A can be absorbed, CO₂ contained in the CO₂-containing flue gas 1001A can be recovered using the circulating absorbent 11 a as the CO₂ absorbent 1002 that is still capable of absorbing CO₂, and the CO₂ concentration in the CO₂ absorbent 1002 can be increased.

Furthermore, in the CO₂ recovery apparatus 10E according to the third embodiment, the circulating absorbent 11 extracted from the middle-stage CO₂ absorbing unit 1010-M into the absorbent circulating line 12D via the absorbent circulating line 12D is cooled in the heat exchanger 13, and the cooled circulating absorbent 11 is supplied to the CO₂ absorbing unit 1010.

Furthermore, in the CO₂ recovery apparatus 10E according to the third embodiment, the absorbent circulating line 12D includes a circulating absorbent extracting line 16 for supplying a part of the circulating absorbent 11 into the upper side of the lower-stage CO₂ absorbing unit 1010-L. Because the circulating absorbent 11 has not reached the saturation level under which CO₂ contained in the CO₂-containing flue gas 1001A can be absorbed, by supplying a part of the circulating absorbent 11 to the upper side of the lower-stage CO₂ absorbing unit 1010-L via the circulating absorbent extracting line 16, the CO₂ absorbent 1002 is allowed to absorb more CO₂, thereby recovering as much CO₂ as possible up to the amount that the CO₂ absorbent 1002 is capable of absorbing CO₂ in the CO₂-containing flue gas 1001A. Therefore, the CO₂ absorbent 1002 is allowed to absorb more CO₂, and thus, the CO₂ concentration in the CO₂ absorbent 1002 can be further increased.

Therefore, according to the third embodiment, the CO₂ absorbent 1002 is extracted from the lower side of the middle-stage CO₂ absorbing unit 1010-M in the absorber 1003 as the circulating absorbent 11, cooled, and then supplied to the upper side of the middle-stage CO₂ absorbing unit 1010-M; and a part of the circulating absorbent 11 is supplied to the upper side of the lower-stage CO₂ absorbing unit 1010-L via the circulating absorbent extracting line 16. In this manner, the CO₂ recovery efficiency of the absorber 1010 is further improved, and the amount of CO₂ recovered per unit amount of the CO₂ absorbent can be further increased. As a result, because the CO₂ concentration in the rich solution 1004 at the bottom 1012 of the absorber 1003 can be further increased, the rich solution 1004 with a further increased CO₂ concentration can be supplied to the regenerator 1005, and the mass flow of the CO₂ absorbent 1002 circulated between the absorber 1003 and the regenerator 1005 can be reduced. In this manner, because the mass flow of the rich solution 1004 supplied to the regenerator 1005 can be reduced, the amount of the supplied steam used in regenerating the CO₂ absorbent 1002 in the regenerator 1005 can be reduced, thereby reducing thermal energy consumption. In this manner, the energy efficiency can be further improved.

Furthermore, while in the CO₂ recovery apparatus 10E according to the third embodiment, the circulating absorbent 11 extracted via the absorbent circulating line 12D is supplied to the upper side of the middle-stage CO₂ absorbing unit 1010-M, the present invention is not limited thereto, and the circulating absorbent 11 may be supplied to the upper side of the upper-stage CO₂ absorbing unit 1010-U, or the circulating absorbent 11 may be supplied to both of the upper-stage CO₂ absorbing unit 1010-U and the middle-stage CO₂ absorbing unit 1010-M.

Fourth Embodiment

A CO₂ recovery apparatus according to a fourth embodiment of the present invention will now be explained with reference to FIG. 9.

FIG. 9 is a schematic of the CO₂ recovery apparatus according to the fourth embodiment. The members that are similar to those according to the first embodiment are given the same reference numerals, and redundant explanations thereof are omitted hereunder.

As illustrated in FIG. 9, in a CO₂ recovery apparatus 10F according to the fourth embodiment, the position from which the CO₂ absorbent 1002 is extracted in the absorber 1003 is changed to the lower side of the lower-stage CO₂ absorbing unit 1010-L, from the lower side of the middle-stage CO₂ absorbing unit 1010-M in the CO₂ recovery apparatus 10E according to the third embodiment illustrated in FIG. 8.

In other words, the CO₂ recovery apparatus 10F according to the fourth embodiment includes an absorbent circulating line 12E for extracting the CO₂ absorbent 1002 from the lower side of the lower-stage CO₂ absorbing unit 1010-L as the circulating absorbent 11, and for supplying the extracted circulating absorbent 11 to the upper side of the middle-stage CO₂ absorbing unit 1010-M, instead of the absorbent circulating line 12D included in the CO₂ recovery apparatus 10E according to the third embodiment illustrated in FIG. 8.

In the CO₂ recovery apparatus 10F according to the fourth embodiment, the CO₂ absorbent 1002 is extracted from the lower side of the lower-stage CO₂ absorbing unit 1010-L as the circulating absorbent 11; the CO₂ absorbent 1002 a extracted from the lower side of the upper-stage CO₂ absorbing unit 1010-U via the absorbent extracting line 15B is merged with the circulating absorbent 11; and the merged circulating absorbent 11 a is supplied to the upper side of the middle-stage CO₂ absorbing unit 1010-M. Because the merged circulating absorbent 11 a has not reached the saturation level under which CO₂ contained in the CO₂-containing flue gas 1001A can be absorbed, by using the merged circulating absorbent 11 a as the CO₂ absorbent 1002 that is still capable of absorbing CO₂, CO₂ contained in the CO₂-containing flue gas 1001A can be recovered, and the CO₂ concentration in the CO₂ absorbent 1002 can be increased.

Furthermore, the CO₂ recovery apparatus 10F according to the fourth embodiment includes an absorbent extracting line 15C for extracting CO₂ absorbent 1002 b from the lower side of the middle-stage CO₂ absorbing unit 1010-M, and for supplying extracted CO₂ absorbent 1002 b to the upper side of the lower-stage CO₂ absorbing unit 1010-L. Because the CO₂ absorbent 1002 b has not reached the saturation level under which CO₂ contained in the CO₂-containing flue gas 1001A can be absorbed, by returning the CO₂ absorbent 1002 b containing CO₂ to the absorber 1003, the CO₂ contained in the CO₂-containing flue gas 1001A can be recovered using the CO₂ absorbent 1002 b that is still capable of absorbing CO₂, and the CO₂ concentration in the CO₂ absorbent 1002 can be increased.

Furthermore, in the CO₂ recovery apparatus 10F according to the fourth embodiment, the absorbent extracting line 15C includes a heat exchanger 17 for cooling the CO₂ absorbent 1002. The CO₂ absorbent 1002 b extracted from the middle-stage CO₂ absorbing unit 1010-M to the absorbent extracting line 15C through the absorbent extracting line 15C is cooled in the heat exchanger 17, and the cooled CO₂ absorbent 1002 b is supplied to the lower-stage CO₂ absorbing unit 1010-L. By supplying the cooled CO₂ absorbent 1002 to the lower-stage CO₂ absorbing unit 1010-L, the balanced partial pressure of the CO₂ absorbent 1002 can be decreased. Therefore, the CO₂ absorbent 1002 is allowed to absorb more CO₂ in the lower-stage CO₂ absorbing unit 1010-L.

Therefore, according to the fourth embodiment, the CO₂ absorbent 1002 is extracted from the lower side of the lower-stage CO₂ absorbing unit 1010-L in the absorber 1003 as the circulating absorbent 11, cooled, and then supplied to the upper side of the middle-stage CO₂ absorbing unit 1010-M in the absorber 1003. The CO₂ absorbent 1002 b extracted from the lower side of the middle-stage CO₂ absorbing unit 1010-M via the absorbent extracting line 15C is cooled, and the cooled CO₂ absorbent 1002 b is again supplied to the CO₂ absorbing unit 1010. In this manner, the CO₂ recovery efficiency of the absorber 1003 is further improved, and the amount of CO₂ recovered per unit amount of the CO₂ absorbent can be further increased. As a result, because the CO₂ concentration in the rich solution 1004 at the bottom 1012 of the absorber 1003 can be further in increased, the rich solution 1004 with a higher CO₂ concentration can be supplied to the regenerator 1005. Thus, the mass flow of the CO₂ absorbent 1002 circulated between the absorber 1003 and the regenerator 1005 can be reduced. In this manner, because the mass flow of the rich solution 1004 supplied to the regenerator 1005 can be reduced, the amount of the supplied steam used in regenerating the CO₂ absorbent 1002 in the regenerator 1005 can be reduced, thereby reducing the thermal energy consumption. In this manner, the energy efficiency can be further improved.

Furthermore, while in the CO₂ recovery apparatus 10F according to the fourth embodiment, the circulating absorbent 11 extracted through the absorbent circulating line 12E is supplied to the upper side of the middle-stage CO₂ absorbing unit 1010-M, the present invention is not limited thereto, and the circulating absorbent 11 may be supplied to the upper side of the upper-stage CO₂ absorbing unit 1010-U, or the circulating absorbent 11 may be supplied to both of the upper-stage CO₂ absorbing unit 1010-U and the middle-stage CO₂ absorbing unit 1010-M.

As described above, the CO₂ recovery apparatus according to the present invention eliminates the need for replacing the conventional CO₂ recovery apparatus and installing a new CO₂ recovery apparatus by simply providing the absorbent extracting line through which the CO₂ absorbent 1002 that has absorbed CO₂ on the rich side is extracted to the exterior and circulated to the lean side of the absorber 1003 to a conventional CO₂ recovery apparatus. Furthermore, depending on the number of stages in the CO₂ absorbing units 1010 included in the absorber 1003, the absorbent extracting line should be designed so as to extract the CO₂ absorbent 1002 that has absorbed CO₂ from the rich side to the exterior as required, and to allow the same to be circulated to the lean side of the absorber 1003. In this manner, by improving the CO₂ recovery efficiency of the absorber 1003, and increasing the CO₂ concentration in the rich solution 1004, the rich solution 1004 with a higher CO₂ concentration can be supplied to the regenerator 1005, so that the amount of the circulation of the CO₂ absorbent 1002 between the absorber 1003 and the regenerator 1005 can be reduced. Therefore, by reducing the mass flow of the rich solution 1004 that is supplied to the regenerator 1005, the amount of the steam required in regenerating the CO₂ absorbent 1002 can be reduced, thereby reducing the thermal energy consumption. In this manner, the energy efficiency can be improved.

Thus, in a large CO₂ recovery apparatus that recovers equal to or more than 1000 tons per day, for example, simply by including the absorbent extracting line for extracting the CO₂ absorbent 1002 and allowing the same to be circulated to the lean side of the absorber 1003 in the conventional CO₂ recovery apparatus, the CO₂ concentration in the rich solution 1004 supplied to the regenerator 1005 can be increased, and the amount of the steam required in the regenerator 1005 can be reduced, thereby reducing the thermal energy, such that further energy savings can be achieved in the process of regenerating the CO₂ absorbent 1002.

INDUSTRIAL APPLICABILITY

As described above, because the CO₂ recovery apparatus and the CO₂ recovery method according to the present invention can increase the CO₂ concentration in the rich solution discharged from the bottom of the absorber, and reduce the mass flow of the CO₂ absorbent circulating between the absorber and the regenerator, the amount of the steam required in regenerating the CO₂ absorbent can be reduced, and the thermal energy can be reduced. Accordingly, when the amount of CO₂ recovered by the CO₂ recovery apparatus increases, by increasing the CO₂ concentration in the rich solution, the amount of the steam required in regeneration can be reduced upon regenerating the CO₂ absorbent, in spite of the increase in the amount of recovered CO₂. Thus, the CO₂ recovery apparatus and the CO₂ recovery method according to the present invention are suited for a CO₂ recovery apparatus and CO₂ recovery control that provides energy savings. 

The invention claimed is:
 1. A CO₂ recovery apparatus comprising: an absorber that brings flue gas containing CO₂ into counter-current contact with CO₂ absorbent to reduce CO₂; a regenerator that regenerates rich solution that has absorbed CO₂ through heat exchange, in which lean solution having CO₂ reduced in the regenerator is reused in the absorber, wherein the absorber includes at least one CO₂ absorbing unit that recovers CO₂ in the flue gas containing CO₂; a washing unit arranged above the at least one CO₂ absorbing unit within the absorber; and a water supplying apparatus configured to supply the washing unit with water from a top thereof; a lean-solution supplying apparatus that is connected to a position between the washing unit and the absorbing unit, and configured to supply the absorber with the lean solution regenerated from the rich solution in the regenerator; and an absorbent circulating line connecting a lower side of the CO₂ absorbing unit to a position between the upper side of the CO₂ absorbing unit and the lower side of the CO₂ absorbing unit without passing through the regenerator, the absorbent circulating line being arranged separately from the lean-solution supplying apparatus, and having a pump that extracts the CO₂ absorbent and a heat exchanger that cools the CO₂ absorbent extracted from the absorber and circulating in the absorbent circulating line.
 2. The CO₂ recovery apparatus according to claim 1, wherein the CO₂ absorbing unit includes an upper-stage CO₂ absorbing unit and a lower-stage CO₂ absorbing unit, and the CO₂ recovery apparatus further comprises: an absorbent extracting line through which CO2 absorbent is extracted from a lower side of the upper-stage CO2 absorbing unit, wherein the absorbent circulating line connects a lower side of the lower-stage CO₂ absorbing unit and an upper side of the lower-stage CO₂ absorbing unit to supply the lower-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit merged with the CO₂ absorbent extracted from the lower side of the upper-stage CO₂ absorbing unit, and the heat exchanger cools the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted via the absorbent extracting line.
 3. The CO₂ recovery apparatus according to claim 1, wherein the CO₂ absorbing unit includes an upper-stage CO₂ absorbing unit, a middle-stage CO₂ absorbing unit, and a lower-stage CO₂ absorbing unit, and the CO₂ recovery apparatus further comprises: an absorbent extracting line through which CO₂ absorbent is extracted from a lower side of the upper-stage CO₂ absorbing unit, wherein the absorbent circulating line connects a lower side of the middle-stage CO₂ absorbing unit and the upper side of the middle-stage CO₂ absorbing unit to supply the middle-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit merged with the CO₂ absorbent extracted from the lower side of the upper-stage CO₂ absorbing unit, and the heat exchanger cools the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the rich side of the upper-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted via the absorbent extracting line.
 4. The CO₂ recovery apparatus according to claim 3, further comprising a circulating absorbent extracting line through which a part of the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit via the absorbent circulating line is supplied to an upper side of the lower-stage CO₂ absorbing unit.
 5. The CO₂ recovery apparatus according to claim 1, wherein the CO₂ absorbing unit includes an upper-stage CO₂ absorbing unit, a middle-stage CO₂ absorbing unit, and a lower-stage CO₂ absorbing unit, and the CO₂ recovery apparatus further comprises: an absorbent extracting line through which CO₂ absorbent extracted from a lower side of the upper-stage CO₂ absorbing unit, wherein the absorbent circulating line connects the lower side of the lower-stage CO₂ absorbing unit and the upper side of the middle-stage CO₂ absorbing unit to supply the middle-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit merged with the CO₂ absorbent extracted from the lower side of the upper-stage CO₂ absorbing unit, and the heat exchanger cools the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit before the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted via the absorbent extracting line.
 6. The CO₂ recovery apparatus according to claim 5, wherein CO₂ absorbent extracted from a lower side of the middle-stage CO₂ absorbing unit is supplied to an upper side of the lower-stage CO₂ absorbing unit.
 7. The CO₂ recovery apparatus according to claim 6, further comprising a heat exchanger that cools the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit.
 8. The CO₂ recovery apparatus according to claim 2, wherein an end of the absorbent extracting line is connected to a rich solution supply line through which the rich solution is supplied to the regenerator, and the rich solution is extracted through the rich solution supply line.
 9. The CO₂ recovery apparatus according to claim 2, wherein the absorbent circulating line is configured to be supplied with a mass flow of the CO₂ absorbent having a mass flow ratio between 0.1 to 6.0 with respect to a mass flow of the rich solution supplied to the rich solution supply line through which the rich solution is supplied to the regenerator.
 10. The CO₂ recovery apparatus according to claim 1, wherein the heat exchanger is configured to cool the CO₂ absorbent extracted from the CO₂ absorbing unit to a temperature of 30 to 50 degrees Celsius.
 11. A CO₂ recovery method comprising the steps of: bringing flue gas containing CO₂ into counter-current contact with CO₂ absorbent in a CO₂ absorbing unit included in an absorber to reduce CO₂; supplying a regenerator with rich solution of the CO₂ absorbent that has absorbed CO₂ and regenerating the rich solution therein into lean solution having CO₂ reduced; supplying the absorber with the lean solution and reusing therein the lean solution; washing CO₂-reduced flue gas by bringing the CO₂-reduced flue gas into counter-current contact with water supplied from a top of a washing unit arranged above the CO₂ absorbing unit within the absorber; supplying the absorber with the lean solution regenerated from the rich solution in the regenerator to a position between the washing unit and the absorbing unit; extracting the CO₂ absorbent that has absorbed CO₂ as circulating absorbent from a lower side of the CO₂ absorbing unit to exterior; cooling the circulating absorbent; supplying the lean solution to a position between the washing unit and the absorbing unit; supplying the circulating absorbent to a position between the upper side of the CO₂ absorbing unit and the lower side of the CO₂ absorbing unit without passing through the regenerator; and circulating the circulating absorbent separately from the lean solution through an absorbent circulating line having a pump that extracts the CO₂ absorbent and a heat exchanger that cools the CO₂ absorbent extracted from the absorber.
 12. The CO₂ recovery method according to claim 11, further comprising: extracting the CO₂ absorbent from a lower side of a lower-stage CO₂ absorbing unit in the absorber including two such CO₂ absorbing units that are an upper-stage CO₂ absorbing unit and the lower-stage CO₂ absorbing unit; merging CO₂ absorbent extracted from a lower side of the upper-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit; cooling the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the lower side of the upper-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit; and supplying the CO₂ absorbent thus merged to an upper side of the lower-stage CO₂ absorbing unit so as to recover CO₂ contained in the flue gas containing CO₂.
 13. The CO₂ recovery method according to claim 11, further comprising: extracting CO₂ absorbent from a lower side of a middle-stage CO₂ absorbing unit in the absorber wherein the CO₂ absorbing unit comprises an upper-stage CO₂ absorbing unit, the middle-stage CO₂ absorbing unit, and a lower-stage CO₂ absorbing unit; merging CO₂ absorbent extracted from a lower side of the upper-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit; cooling the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted from the lower side of the upper-stage CO₂ absorbing unit; and supplying the CO₂ absorbent thus merged to an upper side of the middle-stage CO₂ absorbing unit so as to recover CO₂ contained in the gas containing CO₂.
 14. The CO₂ recovery method according to claim 13, further comprising supplying a part of the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit to an upper side of the lower-stage CO₂ absorbing unit.
 15. The CO₂ recovery method according to claim 11, further comprising: extracting the CO₂ absorbent from a lower side of a lower-stage CO₂ absorbing unit in the absorber wherein the CO₂ absorbing unit comprises an upper-stage CO₂ absorbing unit, a middle-stage CO₂ absorbing unit, and the lower-stage CO₂ absorbing unit; merging CO₂ absorbent extracted from a lower side of the upper-stage CO₂ absorbing unit with the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit; cooling the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit before or after the CO₂ absorbent extracted from the lower side of the upper-stage CO₂ absorbing unit is merged with the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit; and supplying the CO₂ absorbent thus merged to an upper side of the middle-stage CO₂ absorbing unit so as to recover CO₂ contained in the gas containing CO₂.
 16. The CO₂ recovery method according to claim 15, further comprising supplying CO₂ absorbent extracted from a lower side of the middle-stage CO₂ absorbing unit to an upper side of the lower-stage CO₂ absorbing unit.
 17. The CO₂ recovery method according to claim 16, further comprising cooling the CO₂ absorbent extracted from the lower side of the middle-stage CO₂ absorbing unit and thereafter supplying the CO₂ absorbent to the upper side of the lower-stage CO₂ absorbing unit.
 18. The CO₂ recovery method according to claim 12, further comprising: extracting rich solution from a rich solution supply line through which the rich solution is supplied to the regenerator; and circulating the rich solution extracted through the rich solution supply line to the lower side of any one of the upper-stage CO₂ absorbing unit, the middle-stage CO₂ absorbing unit, and the lower-stage CO₂ absorbing unit.
 19. The CO₂ recovery method according to claim 12, further comprising extracting the rich solution, wherein a dividing ratio between a circulation mass flow of the CO₂ absorbent extracted from the lower side of the lower-stage CO₂ absorbing unit to any one of the upper-stage CO₂ absorbing unit, the middle-stage CO₂ absorbing unit, and the lower-stage CO₂ absorbing unit and a mass flow of the rich solution supplied to the regenerator is between 0.1 to 1.0 and 6.0 to 1.0.
 20. The CO₂ recovery method according to claim 12, wherein a temperature of the CO₂ absorbent extracted from the CO₂ absorbing unit after being cooled is 30 to 50 degrees Celsius.
 21. The CO₂ recovery method according to claim 11, wherein a mass flow ratio between a mass flow of the rich solution supplied to the regenerator and an amount of absorbent circulation is optimally controlled depending on a gas temperature of flue gas introduced to the absorber so as to reduce operation costs.
 22. The CO₂ recovery method according to claim 21, wherein, when the CO₂ absorbent has a decreased concentration thereof as a result of extended operation, an amount of circulation in an entire system is increased to maintain a CO₂ absorption capability. 